[PATCH v2] ARM: signal: fix armv7-m build issue in sigreturn_codes.S

[Dave.Martin@arm.com (Dave Martin)]

On Tue, Nov 12, 2013 at 10:57:46PM -0800, Victor Kamensky wrote:
> Hi,
> 
> Here is version 2 of fix to armv7-m build failure in sigreturn_codes.S.
> It is based on Dave's suggestion on [1]. Basically it set
> arch to arm4t explicitly if CONFIG_CPU_THUMBONLY is set.
> That enables both arm and thumb opcodes and code merged with
> the rest of image.
> 
> Version 1 [2] used conditional compilation.
> 
> Fix was tested
>    linux-next with efm32_defconfig build (along with few other fixes)
>    rmk-next BE/LE arndale build/boot and LTP rt_sigaction0? tests run
> 
> Dave, I've added your name with Suggested-by tag, please
> let me know if it is not OK with you, I'll remove it then.
> 
> Uwe, is it possible for you to test that this fix runs on 
> efm32? Sorry, for multiple requests.
> 
> To address concern about fragility of proposed solution
> I looked binutils bfd/elf32-arm.c
> 
>   https://sourceware.org/git/?p=binutils.git;a=blob;f=bfd/elf32-arm.c;h=5af1643bf506870e741ee4da7bd645083619e16d;hb=HEAD
> 
> Attributes merge deals with couple things:
> 
> Tag_CPU_arch: tag_cpu_arch_combine function deals with it
> and from its tables it seems that v4t is compatible with
> any latter version and resulting value will come from 
> latter version. I.e v4t and v7 (v7m) would merge fine.
> 
> Tag_CPU_arch_profile: since in case of '.arch armv4t' 
> profile attribute is not generated it is merged fine
> with any other profile. Unlike in case of 
> '.arch armv7a' and '.arch armv7m' profile values would
> be 'Application' and 'Microcontroller' and those 
> conflict.
> 
> Above logic seems to be universal, so other linkers
> may follow it too. So it seems it is good to use 
> '.arch armv4t' with armv7m code.

[Apologies if you already saw a reply to this.  I was writing one
previously, but I think I never finished it.  Anyway, here it is...]

After thinking about this, I'm not sure that this logic is really sound.

By allowing v7M objects to be linked together with ARM objects, ld
creates objects with nonsensical attributes:

File Attributes
  Tag_CPU_name: "7-M"
  Tag_CPU_arch: v7
  Tag_CPU_arch_profile: Microcontroller
  Tag_ARM_ISA_use: Yes
  Tag_THUMB_ISA_use: Thumb-2

i.e., a object cannot really be compatible with the microcontroller
profile and also contain ARM instructions.  v7-M is absolutely not a
superset of v4T.


So, I think we are getting lucky here, and there's no real reason why
a different linker (or future versions of GNU ld) should allow this.

The alternative is to use #ifdefs, and replace the ARM instructions and
associates directives with suitable .space directives or nops in the ARM
case.

This brings us back closer to your original suggestion, I guess.


Cheers
---Dave